Injection systems and in particular leak-oil-free common-rail injection systems require a control element such as, for example, a piezoelectric actuator in the high-pressure chamber. In order to ensure the operational capability of the piezoelectric actuator even under high pressures up to over 2000 bar, the pressure must also be able to act laterally on the piezoelectric stack or, as the case may be, piezoelectric ceramic body so as to assist the expansion capability of the piezoelectric stack of the piezoelectric actuator.
A piezoelectric actuator of the aforesaid kind is described for example in WO 02/061856 A1. In that case the ceramic body of said piezoelectric actuator is enclosed by means of a polymer or plastic sleeve. However, a hermetic sealing of the ceramic body against the fuel subject to a high pressure such as for example 2000 bar is hardly feasible or not feasible with the plastics known on the filing date of the present patent application. Due to an occasionally unavoidable electrical conductivity of commercially available fuels, for example due to a small acid content, even slight wetting of the piezoelectric ceramic can lead to voltage flashovers between the internal electrodes of the piezoelectric actuator. Moreover, high expansions of the plastic encapsulation which exacerbate this problem occur at the polarization cracks. WO 02/061856 also describes the use of a filler material between the piezoelectric stack and the polymer or plastic sleeve. In the case of the described filler material there is, however, the problem that during an expansion of the piezoelectric stack the filler material can flow into resulting spaces or joints and be destroyed in the event of a movement of the piezoelectric stack in the opposite direction. Thus, the filler material will be used up or destroyed over the service life of the piezoelectric actuator. The reduced filler material nevertheless means that the pressure being applied externally to the piezoelectric actuator can no longer be transferred efficiently onto the piezoelectric stack.
Also known to the applicant for the purpose of transferring the pressure onto the piezoelectric stack of the piezoelectric actuator is a solution having a hermetically sealed metal sleeve which transfers the hydraulic forces by means of a fluid, such as a silicone oil for example, onto the side surfaces of the piezoelectric ceramic body. However, this solution known internally to the applicant has the disadvantage that the coefficient of thermal expansion of silicone oil is orders of magnitude greater than that of the metal sleeve.
As a result, in particular the tensile stresses in the metal sleeve increase if there is an increase in temperature. The tensile stresses could be reduced by suitable moldings of the expansion folds of the metal sleeve or, as the case may be, metal bellows in the longitudinal and transverse directions. However, with a square-shaped piezoelectric stack, which is cheaper by comparison with piezoelectric stacks having other cross-sectional shapes, the stresses nonetheless increase to an unacceptably high level due to the relatively large volume of fluid. Since the ceramic material of the piezoelectric stack possesses a very low thermal expansion, it is also known internally to the applicant to make the volume of the piezoelectric stack as large as possible compared to the filling. For that purpose the piezoelectric stack could be embodied as a circular cylindrical shape for example. The circular cylindrical embodiment of the piezoelectric stack is considerably more complicated and labor-intensive in manufacturing terms, however, and consequently significantly more cost-intensive than a square-shaped piezoelectric stack.